1. Field of the Invention
This invention relates to free floating disc valves; and, in particular, to a positive closure arrangement for such a valve.
2. Description of the Prior Art
A nuclear steam turbine power plant generally consists of a closed-loop series connection comprising a steam generator element, a high pressure turbine element, a moisture separator and reheater element, a low pressure turbine element and a condenser. Between the steam generator element and the high pressure turbine there is disposed a main turbine stop valve and a main turbine throttle valve. These valves control the flow of high pressure, high temperature motive fluid produced within the steam generator element into the remainder of the series connected power plant loop.
The main stop valve and throttle valve are utilized in emergency situations to interdict the flow of steam into the rest of the power plant system. By rapidly closing the stop and throttle valves no further quantity of motive fluid is permitted to enter the turbine elements downstream of the valves. However, steam which has already passed through the main turbine stop and throttle valves and expands through the system has the possibility of overspeeding the turbine elements within the power plant. Thus, if an emergency situation, such as a loss of load condition were to develop, the main turbine stop and throttle valves would close, but steam trapped in the system must still expand therethrough into the condenser. The passage of steam through the low pressure turbine element would, for example, only exacerbate an overspeed condition and increase the possibility of turbine failure.
For this purpose, at least one steam pressure relief, or dump, valve is disposed between the moisture separator reheater element and the low pressure turbine. The dump valve permits venting of steam trapped downstream of the main turbine stop and throttle valves directly to the condenser element or into atmosphere.
The dump valve is usually a free floating disc type valve which is activated by controlling pressures in a pressure chamber above the disc. When the disc is closed, a pilot valve completely seals the pressure chamber and permits equalization of pressures above and below the disc. Decreasing the pressure above the disc, by the expedient of venting the pressure chamber to atmosphere to the condenser, permits the high pressure steam on the inlet side of the valve disc to exert a force tending to raise the valve disc. Thus, steam passes through the dump valve either to the condenser or to atmosphere.
It is possible, however, for friction between a seal arrangement surrounding the valve disc and the interior portions of the chamber housing to prevent the valve disc from responding to a closing signal impressed thereon by suitable governor means. The usual method of closing dump valves is to re-isolate the pressure chamber by closing of the pilot valve and letting the pressures above and below the valve disc equalize. The weight of the valve, plus any force unbalance due to a difference in effective areas on the upper and lower surfaces of the disc, are usually sufficient to overcome the frictional force generated between the seals and the valve chamber walls.
However, when utilizing a prior art free floating disc valve in a nuclear steam power plant which generally operates at lower steam pressures than the conventional fossil fuel plant, reliance upon the weight of the disc and the pressure unbalance is not always sufficent to overcome the frictional force. Thus, the free floating disc within the dump valve may not predictably re-seat and disruption to the entire power plant system will occur. It is therefore advantageous to provide a positive closure system for a free floating disc type valve to insure proper seating of the valve disc when the pressure relief function has been performed.